A catalyst containing cobalt oxide and zinc oxide for use in the synthesis of C1-C3 aliphatic hydrocarbons is known from U.S. Pat. No. 4,039,302.
WO-A-03/090925 describes a catalyst comprising particles of a cobalt and zinc co-precipitate, said particles having a volume average particle size of less than 150 μm and a particle size distribution wherein at least 90% of the volume of the catalyst particles have a size between 0.4 and 2.5 times the average particle size. Although this catalyst is an improvement over the previously known catalysts, there is still room for improvement, especially with respect to the balance of properties required between the properties needed during activation on the one hand and attrition and activity properties during FT operation on the other hand.
In the case of slurry-phase Fischer Tropsch catalysts, always a good balance should be adjusted between activity (Fischer Tropsch performance) and separation properties. In that respect, the (powder) catalyst should have an optimized particle size distribution giving good mass-transfer and separation properties. These properties should not change substantially during time on stream in the process so that activity and separation properties are retained over long periods of time.
Prior art supported cobalt catalysts for Fischer Tropsch are generally based on a mechanically strong support that is impregnated with a cobalt solution and subsequently dried, calcined and reduced. Alternatively, a mechanically strong cobalt-support system can be precipitated by optimizing the precipitation conditions. Such formulations are mechanically stable in both the fluid-bed catalyst activation and the slurry-phase Fischer Tropsch reactor, although attrition in the Fischer Tropsch reactor may lead to fines (submicron particles) formation and consequently separation problems. The preferred particle size distribution to achieve good fluidization behaviour during fluid-bed activation is generally different from the required, preferred particle size distribution for the Fischer Tropsch process. The chosen particle size of the fresh catalyst always forms a compromise between the two.
In the various patents it has been disclosed that the catalyst can consist of cobalt and zinc oxide. It is, however, also indicated that the catalyst contains one or more other components, such as one or more promoters. Without further exemplification promotors such as ruthenium, hafnium, platinum, zirconium, palladium, rhenium, cerium, or lanthanum have been mentioned, typically in a cobalt to promoter atomic ratio of up to 10:1. No effect of the use of promotors has been disclosed in those patents.
In view of the requirement of prolonged use, also under severe conditions, there is a need for a catalyst having improved mechanical strength, or (the result thereof), less attrition.
The present invention is based on the surprising discovery that the use of zirconium(IV)oxide and/or aluminiumoxide as promoter, in specific, limited amounts, in a cobalt zinc oxide FT-catalyst, leads to improved mechanical strength.
The invention is accordingly directed to a catalyst suitable for catalyzing a Fischer-Tropsch reaction, said catalyst comprising cobalt metal supported on zinc-oxide and an amount of zirconium(IV)oxide and/or aluminiumoxide of between 0.5 and 2.5 wt. % calculated as metal, based on the weight of the calcined catalyst.
In the catalyst of the invention both aluminium oxide and zirconium oxide may be used, but it is preferred to use zirconium.
In a preferred embodiment, the amount of aluminium oxide and/or zirconium oxide is between 0.8 and 2 wt. %, as defined above. It has been found that with the specified amounts of these promotors, in a standard attrition test, the average particle size is maintained, and, in combination, the amount of fines is reduced, as is shown in the examples.
A catalyst according to the present invention shows a marked decrease in the amount of fines caused in a standard attrition test, said amount of fines being characterized by the Δ−minus 5 μm, i.e. the increase of the amount of particles below 5 μm. This value is generally below 7.5%. The test is defined in the Examples.
A further characterization of the low attrition is that the decrease of the average particle size, defined as the decrease of the d[v,0.5]−value, or Δ−d[v,0.5], is below 2.5 μm, where d[v,0.5] is the size below which 50% of the particle volume exists in a volume distribution of the particle size distribution curve. This number typically applies for a powder catalyst with an initial average particle size (d[v,0.5]) in the range of up to 75 μm prior to the standard attrition test.
A catalyst according to the invention has been found to be particularly favourable for use in a stirred slurry-phase reactor, bubble-column reactor, loop reactor or fluid-bed reactor.
A catalyst according to the invention shows very favourable separation properties and can for example very suitably be separated from the reaction mixture by filtration.
A catalyst according to the invention has an extremely good balance between activity and separation properties
Preferably the volume average particle size of the catalyst is less than 75 μm, more preferably less than 50 μm. The lower limit is not particular critical, provided it meets the criteria of the invention. For practical purposes it is preferred that the size is at least such that the particles can still be separated from a liquid reaction mixture. Particularly suitable is for example a catalyst with a volume average particle size of 2 μm or more. Very good results have been achieved with a catalyst having a volume average particle size in the range of 2-50 μm.
The volume average particle size and particle size distribution have been determined using a Malvern MS2000 apparatus, on which particle size distributions in the range 0.01 μm to 2000 μm can be measured. The method is based on a laser diffraction technique with the solid (powder) dispersed in water. The slurry is pumped through a measuring cell, where the scattering profile of the laser beam through the cell is recorded. The scattering profile is subsequently converted to the volume particle size distribution of the powder in the slurry, using the Malvern equipment software. No ultrasonic treatment was applied prior to the measurement.
Preferably the pore volume of the catalyst—as determined by nitrogen adsorption (N2-BET), measured on an Ankersmit Quantachrome Autosorb-6 apparatus, after degassing the sample at 180° C. to a pressure of 3.3 Pa (25 mTorr)—is at least mainly formed by pores having a diameter in the range of 5-100 nm. Much preferred wherein there are essentially no pores with a diameter of less than 5 nm (in particular less than 5% of the pore volume formed by pores with a diameter of less than 5 nm). It has been found that such a catalyst has particularly good diffusion properties for reactant and product. Such a catalyst has also been found to be highly selective in C5+ production in a Fischer-Tropsch process.
Very good results have been achieved with a catalyst having a pore volume of less than 0.6 ml/g. The pore volume is preferably at least 0.05 ml/g. Particularly suitable is a catalyst with an pore volume of less than 0.5 ml/g.
Such a catalyst has been found to have particularly good physical strength properties, which is advantageous in applications in various types of reactors, including slurry-phase reactors, loop-reactors, bubble-column reactors and fluid-bed reactors.
The catalyst of the invention is based on cobalt on zinc oxide. The composition of the catalyst can be varied widely, which composition the skilled professional will know to determine, depending upon the intended purpose. Preferably, the zinc to cobalt atomic ratio is in the range of 75 to 0.1, preferably of 40 to 0.1 and more preferably in the range of 20 to 0.3.
Very good results have been obtained with a catalyst according to the invention which is essentially free of sodium. It has been found that a catalyst containing a relatively high amount of sodium is reduced in strength. Further, the presence of sodium has been found to be detrimental to the activity of the catalyst, reducing its Fischer-Tropsch activity. Therefore, a catalyst with a sodium content of less than 0.5 wt. %, more in particular of 0 to 0.15 wt. %, even more in particular of 0 to 0.05 wt. % based upon the weight of the catalyst, is preferred.
Very good results have been achieved with a catalyst according to the invention having a low content of copper or being essentially free of copper. Copper may stimulate side reactions, such as the formation of an alcohol by hydrogenation of a ketone, an aldehyde or a carboxylic acid, which are usually preferably avoided or suppressed, especially in a Fischer-Tropsch process. The copper content is preferably less than 2 wt. %, more preferably 0 to 0.5 wt % even more preferably 0 to 0.2 wt. %, based upon the weight of the catalyst.
The preparation of the catalyst of the present invention comprising cobalt and zinc oxide and the aluminium oxide and/or zirconium(IV)oxide promotor, can be done by co-precipitation of cobalt and zinc ions, together with the promoter ions.
In order to produce the catalyst with the specific structure, various options are available.
A very suitable method is based on the method described in WO 03/090925, whereby the process is modified in such a way that the structure of the present invention is obtained. This can, for example, be done by changing the stirring speed or the rate of injection of the solution(s) in the reaction vessel or by applying other types of agitation. As the particles of the catalyst of the present invention are generally smaller and have a different particle size distribution, compared to said document, higher stirring speeds, shorter contact time and/or higher injection speeds are preferred.
In the alternative one may produce first particles that are within the ranges of WO 03/090925 following which the particles are comminuted to obtain the catalyst of the invention, for example by ultrasonic treatment or another comminuting treatment.
Generally the process comprises a coprecipitation method, wherein an acidic solution comprising zinc ions and cobalt ions and an alkaline solution are supplied to a reactor comprising an aqueous medium, preferably water or an aqueous solution, wherein the acidic solution and alkaline solution are contacted in the aqueous medium and a precipitate comprising cobalt and zinc is formed. The precipitate is thereafter separated from the aqueous medium (which may have formed a slurry together with the precipitate). The separated cobalt and zinc comprising precipitate is then dried and may be post-treated, e.g. calcined, etc, to form said catalyst.
The promoter ions are preferably included in the acidic solution. As precursors for the promoters, promotor ion salts, soluble in an acidic (pH<5) are preferably used, such as zirconyl(IV) nitrate or ZrCl4 in the case of zirconium promoted catalysts, or hydrated aluminium sulphate for preparing aluminum promoted catalysts.
The combination of acidic solution and alkaline solution is preferably chosen such that the components of the acidic solution and of the alkaline solution are soluble in the aqueous medium, but that the cobalt and zinc and possible promoter precipitate when they are contacted with the alkaline solution, while the counter ions of zinc, cobalt and possible promoter substantially remain in solution. The skilled professional will know how to choose appropriate conditions, such as the type of counter ions and the concentrations for each of the components.
This method has been found to be particularly suitable for preparing a catalyst as described above.
Preferably the precipitation of particles is carried out at a substantially constant pH, in particular at a pH value varying at most ±0.2 pH units around a set-point value. Thus it has been found possible to make a catalyst precursor with very favourable free flowing characteristics and good mechanical strength properties.
Preferably, the alkaline solution and the acidic solution are supplied to the reactor simultaneously (from separate conduits).
Optionally the cobalt in the isolated and dried precipitate or calcined product is reduced to metallic cobalt
Suitable sources for ionic zinc respectively ionic cobalt include salts thereof that are soluble in the acidic solution and in water in a sufficient concentration. Preferred examples of such salts include zinc nitrate respectively cobalt nitrate and zinc acetate respectively cobalt acetate and other inorganic or organic salts of cobalt respectively zinc that have a similar solubility in the acidic solution
Suitable components for co-precipitating with the cobalt ions and zinc ions present are inorganic salts and organic salts that are soluble in an aqueous alkaline solution in a sufficient concentration, such as hydroxides, carbonates, urea, isocyanates and any other salt that can be used as base source and that can be dissolved water of in the alkaline solution. Preferred examples of such salts include ammonium carbonate, ammonium bicarbonate and other inorganic or organic salts of carbonate that have at least a similar solubility in the alkaline solution.
The pH of the acid solution is preferably in the range of 1-5. The pH of the alkaline solution is preferably in the range of 6-14. The pH in the aqueous medium (wherein the co-precipitation takes place) is preferably in the range of 4-9, depending upon the type of precursor salts used as a source for cobalt, zinc and alkaline component(s).
The temperature during the co-precipitation process is preferably chosen in the range of 5-98° C., more preferably in the range of 25-80° C.
The present invention further relates to the use of a catalyst according to the invention in a slurry reactor, a loop reactor, a bubble-column reactor or a fluid-bed reactor. The present invention further relates to the use of a catalyst according to the invention in a Fischer-Tropsch process or a functional group hydrogenation process, such as nitrile hydrogenation to amines.